The present invention relates generally to a two stage control circuit for air conditioning applications and, more particularly, to a circuit for controlling room temperature thermostat and demand defrost functions of a reversible air cycle heat pump.
While various aspects and features of the present invention are adaptable to a variety of air conditioner and heat pump units, the invention is particularly adapted for the control of an air valve heat pump wherein, to alternately provide heating and cooling modes of operation, indoor and outdoor airflow is redirected across the condensor and evaporator, rather than by reversing the function of the evaporator and condensor heat exchangers as is more conventional in heat pump practice. In particular, while the unit is operating in the cooling mode, outdoor air circulates in heat exchange relationship with the condensor, and indoor air circulates in heat exchange relationship with the evaporator. Conversely, during heating mode operation, outdoor air circulates in heat exchange relationship with the evaporator, and indoor air circulates in heat exchange relationship with the condensor. A commonly-assigned U.S. patent application Ser. No. 144,796, filed Apr. 28, 1980, by McCarty et al, and entitled "AIR VALVE HEAT PUMP" is directed to such a heat pump, to which reference may be had for further information.
While various arrangements may be employed to defrost the evaporator of such a heat pump, a presently prefered arrangement involves a passive defrosting system wherein, when defrosting is required, operation of the refrigerant compressor is interrupted, and refrigerant pressure within the system is allowed to equalize, with attendant equalization of temperature. In this way, heat from various elements of the system is allowed to reach the evaporator, melting the frost therefrom. Various valving arrangements may be employed to hasten and augment this process. Such a defrosting arrangement is the subject matter of another commonly-assigned U.S. patent application Ser. No. 144,795, filed Apr. 28, 1980, by McCarty, now U.S. Pat. No. 4,285,210 and entitled "SELF-CONTAINED HEATING AND COOLING APPARATUS", to which reference may be had for further information.
By way of example, a reversible heat pump for which the circuit of the present invention is particularly intended comprises a single room-sized unit mountable in an opening through an outdoor wall of a building, the single unit including all major components, namely the evaporator, condenser, compressor, fans, auxillary electric resistance heaters, as well as a major portion of the present control circuitry.